Heretofore, thermoplastic coated paperboard cartons having flat end closures of the in-folded and/or out-folded fin sealed types have been used with generally satisfactory results. When the, so-called, in-folded type of end closure is used, several panel thickness are operative in the folding operations. The multiple panel thicknesses, when folded, cause causing stretching or tensioning of the outer layers around the inner layers to the extent that the outermost thermoplastic layers may tend to pull apart and crack, thereby destroying the liquid impermeable thermoplastic layer, permitting leaking therethrough.
Two known approaches involving supplemental score lines to solve such a cracking problem are shown in Holmstrom U.S. Pat. No. 4,267,957 and in Lisiecki U.S. patent application Ser. No. 634,718.